1. Field of the Invention
The present invention generally relates to image processing apparatuses, and particularly relates to an image processing apparatus which reduces moiré that is generated when a screen dot image is scanned as an input document.
2. Description of the Related Art
In digital copiers, moiré (density fluctuation) appears when a screen dot document is input, resulting in the degradation of image quality. Factors that cause moiré include 1) interference between the screen dot pitches of the document and the sampling interval at the time of image scans that generate each pixel value, and 2) non-linier processing such as the saturation of density levels caused by spatial filtering or steep changes in input/output values caused by γ processing.
A method that suppresses the generation of moiré caused by the first factor includes blurring the input image data by use of a smoothing filter. A method that suppresses the generation of moiré caused by the second factor includes employing weak spatial filtering so as to avoid the saturation of image data or employing smooth γ processing.
The related-art methods as described above can properly reduce moiré relating to a screen dot image, but gives rise to a problem in that line drawings such as letters are blurred so as to result in the degradation of image resolution and sharpness. It is possible to obtain images having less moiré by increasing the sampling interval used at the time of image scans. This creates a problem, however, in that a CCD-based scanning device having high resolution is necessary, thereby increasing costs.
Accordingly, there is a need for an image processing apparatus that can effectively suppress moiré relating to a screen dot image.